Michelle Byrne Walsh is an editor at State-by-State Gardening, a master gardener and a member of Garden Writers Association.

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How to Build a Cold Frame by Michelle Byrne Walsh

A cold frame can help you harden off spring transplants, get a head start on the growing season, and lengthen the fall season for cold-tolerant edibles such as lettuce, kale, radishes and herbs by one to three months. Here’s how to make one from a window and some lumber.

You have probably heard this a hundred times: "Harden off seedling plants two weeks before transplanting by moving them to a protected area outdoors or by placing them in a cold frame." But you don't have a cold frame, and perhaps never thought you needed one.

Do you need a cold frame?

In the Midwest, a cold frame can help you harden off transplants, get a head start on the growing season, and lengthen the fall season for cold-tolerant edibles such as lettuce, kale, radishes and herbs by one to three months. Some gardeners even use their cold frames to force bulbs, store root vegetables and propagate hardwood cuttings.

In its purest sense, a cold frame is a structure that provides warmth and light from the sun and blocks the wind. The sun's rays enter through a transparent cover, creating a greenhouse effect that heats the interior. A cold frame outfitted with a heating system, like heating cables, light bulbs or even heat-producing rotting manure, is called a hot bed.

Beyond this basic definition, a cold frame can be constructed from many different materials. The most inexpensive cold frame can be made with four bales of hay and an old window, sheet of plastic or Plexiglas. You assemble the hay bales to create a rectangular hole in the center and place the window or plastic on top.

Kits to create tiny hoop houses or even mini greenhouses can be purchased from garden centers or online retailers. These will act like cold frames, too. To make a miniature hoop house, for example, you can place bent rods over a raised bed, secure the rods into the soil, and cover them with plastic sheeting. There are numerous types and thicknesses of plastic sheeting, most measured in "mils" such as 4 mil or 6 mil (one mil equals one thousandth of an inch). Some mini greenhouse kits feature a series of steel frames with hard plastic sides and a transparent top that opens and closes for access and ventilation.

That's another feature cold frames should have — a means to let overly warm air escape. Even though the air temperature outdoors can be in the 40s and 50s F, the interior temperature of a cold frame can get too hot for the plants. Having a way to ventilate the space will ensure plants and seedlings don't bake. Some cold frames have hinged coverings or windows or cut outs in the plastic sheeting that you have to manually open and close. Others feature temperature-triggered vents or hinges that automatically open and close to let out excess heat at a certain temperature.

Most "permanent" cold frames are built with wood and some type of glass or plastic top. These are desirable because they can withstand years of sun and weather. So that's what I wanted to build. To keep it as inexpensive as possible, I chose to make it from plywood and an old double-paned window that measured 29 by 33 inches. (Believe it or not I actually had an old replacement window in the basement (in case of emergency, aka baseballs). If you don't have windows lying around, you can buy Plexiglas sheets or shop for discontinued or "seconds" windows. You can also visit websites like www.craigslist.org or www.freecycle.org to look for cheap or free materials. Several online greenhouse suppliers sell sheets of double-walled, triple-walled or quad-walled polycarbonate or high-density polyethylene, which you can cut to fit.

Using a window will force you to adhere to a smaller size--probably between 2 to 4 feet. I liked the size my window required us to stick to — 29 by 33 inches — big enough for a couple flats and some pots, small enough to be portable and fit inside the raised beds as well as the tool shed. Portable cold frames allow you to move the frame as the seasons change. I hoped to use it to protect lettuce in my raised beds during the late fall as well as to harden off seedlings in the spring.

When assembled and in use, ideally, cold frames should be located against a south or east wall near the building foundation to take advantage of its heat. If you prefer, you can turn a cold frame into a hot bed by adding heating cables. The bottom heat of a hot bed encourages root growth in plants. Hot beds are useful for seedlings that require constantly warm soil temperatures to germinate.

Decide the dimensions of the box. Because our window measured 29 by 33 inches, we made the box those dimensions. And because the window should slope southward to take advantage of the low angled sunlight, we made the front of the box 6 inches tall and the rear wall 18 inches tall.

Measure and trace the shape of your pieces on the plywood. Templates or patterns might be helpful. And do the math for the sloped sides! Because your window will rest on the top of all four walls, be sure the sloped sides are the same length of the window (the bottom of the side walls' measurements will be different).

Cut the wood walls.

Glue and screw the walls together. We chose butt joints, but if you are a skilled woodworker and are using solid wood boards (versus plywood), other types of joints might be desirable.

Remove the window's existing hardware, if needed. Attach the window to the frame using door hinges. Here we reused the existing hardware holes drilled into the window's frame.

Paint the box. You can choose a color to match your house, as done here, but be sure to paint the interior of the box white for maximum light diffusion.

Attach pipe insulation to bottom of frame--this will protect the wood from the soil and help "seal" the frame to the ground. If needed, attach peel-and-stick door/window insulation to areas where window doesn't seal tightly.

Fashion a sturdy prop stick. I used scrap wood and covered the top with left over insulation foam.

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